The present invention pertains to an improved performance thin film matrix, method of making the thin film matrix and matrix displays incorporating an improved transistor. More particularly, the present invention is directed to a method of making multilayer thin film matrices to increase the yield of the finished matrix devices made therefrom and increase matrix scalability and performance.
In recent years there has been growing interest in thin film transistors (TFT's) and matrix devices incorporating such thin film transistors, such as memory arrays, all types of integrated circuits and replacements for mechanical switches and relays. For example, reed relays can fatigue and MOS switches exhibit too much leakage current.
A specific exemplary use of the thin film matrix transistor is in flat panel displays, such as those which employ liquid crystals, electrochromic or electroluminescense, as replacements for conventional cathode ray tubes (CRT's). The flat panel displays promise lighter weight, less bulk and substantially lower power consumption than CRT's. Also, as a consequence of their mode of operation, CRT's nearly always suffer from some distortion. The CRT functions by projecting an electron beam onto a phosphor-coated screen. The beam will cause the spot on which it is focused to glow with an intensity proportional to the intensity of the beam. The display is created by the constantly moving beam causing different spots on the screen to glow with different intensities. Because the electron beam travels a further distance from its stationary source to the edge of the screen than it does to the middle, the beam strikes various points on the screen at different angles with resulting variation in spot size and shape (i.e. distortion).
Flat panel displays are inherently free of such distortion, because each pixel is photolithographically patterned on the substrate as opposed to being defined by where the CRT electron beam strikes the phosphor on the screen. In the manufacture of the flat panel displays the circuit elements are deposited and patterned, generally by photolithography, on a substrate, such as glass. The elements are deposited and etched in stages to build a device having a matrix of perpendicular rows and columns of circuit control lines with a pixel contact and control element between the control line rows and columns. The pixel contact has a medium thereon which is a substance that either glows (emissive) or modulates the transmission of ambient light (non-emissive) when a threshold voltage is applied across the medium control element. The medium can be a liquid crystal, electroluminescent or electrochromic materials such as zinc sulfide, a gas plasma of, for example, neon and argon, a dichroic dye, or such other appropriate material or device as will luminesce or otherwise change optical properties in response to the application of voltage thereto. Light is generated or other optical changes occur in the medium in response to the proper voltage applied thereto. The optically active medium on each contact is generally referred to as a picture element or "pixel".
The circuitry for a flat panel display is generally designed such that data is generally shifted in on all the column lines each to a predetermined voltage. The conductivity, integrity and reliability of the row and column lines in flat panel and other matrix devices is critical. High conductivity lines are utilized in field emission devices (FED's) and other matrix devices, such as active matrix devices. In an active matrix device, one row is energized to turn on all the transistors in that row (one row is written at a time). That row is then shut off and the data for the next row is shifted into 11 the column lines and then the second row is energized and written. This process is repeated until all the rows have been addressed. All the rows are generally written in one frame period, typically about 1/60th of a second or about 16.7 ms. Then voltages representing the data are supplied selectively to particular columns to cause selected pixels to light up or change optical properties as the row is written. The pixels can be made to change intensity by applying a large voltage or current or a longer pulse of voltage or current. Utilizing liquid crystal display (LCD's) with twisted nematic active material, the display is substantially transparent when not activated and becomes light absorbing when activated or vice versa depending upon polarizer orientation. Thus, the image is created on the display by sequentially activating the pixels, row by row across the display matrix. The geometric distortion described above with respect to CRT's is not a factor in flat panel displays since each pixel location is photolithographically determined and fixed.
One of the major problems that arises with respect to the prior art method of manufacturing structures for active matrix displays (e.g. those employing nonlinear control elements, such as thin film diodes, MIM's or thin film transistors at each pixel) is that they generally suffer production yield problems similar to those of integrated circuits. That is, the yields of devices produced are generally not 100% and the yield (percentage of devices with no defects) can be 0% in a worst case. High quality displays will not tolerate any defective control elements or other components. Also, larger size displays are generally more desirable than smaller size displays. Thus, a manufacturer is faced with the dilemma of preferring to manufacture larger size and/or higher resolution displays, but having to discard the entire product if more than a few transistors and hence if more than a few pixels are defective. In other words, the manufacturer suffers a radically increased manufacturing cost per unit resulting from decreasing usable product yield.
One desirable matrix transistor includes an inverted gate formed on a matrix substrate. In inverted gate TFT LCD structures, the gate metal is generally deposited directly onto a glass substrate. To provide a high performance TFT arrays, the gate metal should have high conductivity, good adhesion to both the substrate and subsequent layers, not form hillocks during the subsequent required high temperature processing, have a minimal step coverage over the gate metal for subsequent layers and the gate metal should be capable of being anodized.
In an attempt to solve these problems, a number of solutions have been attempted, including a single refractory metal layer gate, an aluminum or aluminum/silicon and/or copper alloy gate metal, combinations of a refractory metal and an aluminum gate metal and even a thick etched single layer metal gate. Each of these attempted solutions includes one or more problems as described hereinafter.
These problems of increased cost and decreased yield are dramatically improved in the present invention by providing a method of manufacturing improved matrices including inverted gate matrix transistors having a high performance multilayer gate metal structure with a greatly reduced number of matrix defects which can be utilized in all types of integrated circuits, such as active matrix displays.